Temperature control system for electric heating devices



1944- w. K. KEARSLEY 2,354,918

TEMPERATURE CONTROL SYSTEM FOR ELECTRIC HEATING DEVICES Filed Aug. 26, 1942 A/AvM/H/ .mmmm lmlw laiwiiiai- Inventor: William K. Kearsle g, b g WQ M/V tA "ZM His Attorneg.

Patented Aug. 1 1944 g William K. Kearsley, Schenectady, N. I, assignor to General Electric Company, a corporation of v New York Application August 26, 1942, serialNo. 456,190

a Claims. (Cl. 219-46) My invention relates to temperature control systems for electric heating devices such as electric blankets, electric heating pads, and the like.

Heretofore, in order to protect an electric heating device, such as an electric blanket, against overheating in case it is folded or rolled while in use, in such a manner as to produce an abnormally high temperature therein under normal operating voltage conditions it has been common practice to locate within the blanket a number of thermostats which are so connected in the heating circuit of the blanket as to interrupt this circuit in response to predetermined abnormally high blanket temperatures.

One object of my present invention is'to provide an improved arrangement of apparatus for protecting an electric heating device, such as an electric blanket or the like, which does not require the use of such thermostats or any other auxiliary devices within the device itself and which is not aifected by blanket temperature changes produced by changes in the voltage of the source supplying current to the device.

In accordance with my invention, I provide an arrangement of apparatus which depends for its operation upon the change in the resistance in the heating circuit of the device produced by abnormally high temperatures of the device under normal voltage conditions of the supply circuit and which is not ail'ected by changes in the resistance of the heating circuit produced by abnormally high temperatures resulting from abnormal changes in the voltage of the supply circuit.

My invention will be better understood from the following description when taken in connection with the accompanying drawing, the single figure of which diagrammatically illustrates an electric blanket temperature control system embodying my invention and with certain of the apparatus shown in perspective, and the scope of my invention will be pointed out in the appended claims. 7

Referring to the drawing, ill represents an electric blanket which is controlled by means of a suitable temperature responsive device II that is thermally removed from the blanket and arranged to respond to the ambient temperature of the blanket. The blanket I is provided with a heating element l2 which is preferably arranged in a plurality of convolutions so as to be distributed substantially uniformly throughout the area of the blanket. The ends of the heating element l2 are connected to the terminal pins II and II, respectively, which are arranged to be engaged by a pair or contacts I4 and I4 in a 9915! P us I! when the plugis applied as shown in the drawing. The contact I! of the plug it is permanently connected to the secondary terminal it of a step-down voltage transformer II, the primary. winding of which is arranged to be connected to a suitable supply circuit It by means of a manually controlled switch l9. The contact it of the supply plug I5 is arranged to be connected to the other secondary terminal I6"o1 the transformer IT .by means of the temperature responsive device ll. As shown, the temperature responsive device ll comprises a twoposition snap-action switch 20 having a movable contact 2| adapted in one position of the switch to engage a stationary contact 22 and in the other position of the switch tolengage a station'- ary contact 23. The movable contact 2|, which is permanently connected electrically to the secondary terminal it of the transformer I1, is normally biased into engagement with the stationary contact 22, which is permanently connected electrically to the contact ll of the supply plug I5 so that when the contacts 2i and 22 of the switch are in engagement, the heating circuit of the electric blanket I0 is completed.

The movable contact 2i of the switch 2|! is arranged to be moved out of engagement with stationary contact 22 and into engagement with stationary contact 23 by means of a plunger pin 24, which in turn is controlled by means of a bimetallic element 25. As shown, the bimetallic element-25 is of hairpin or U shape and is supported by means of a stationary pin 26 over which the bimetallic element is draped. The pin 26 is provided with a head 21 to {prevent axial movementof the bimetallic element 25 along pin 26. Asecond stationary pin 28 is also provided above the pin 26 to' prevent vertical movement of the bimetallic element 25 with respect to the pin 26. The bimetallic element 25 is so arranged that when its temperature increases above a predetermined value, its two arms spread apart, whereas when its temperature decreases, its two arms move toward each One of the arms 01' the bimetallic eleother. ment 25 bears against the plunger pin 24 while the other arm bears against an adjustable stop 29. Therefore, when the temperature of the 'bi-,

metallic element 25 exceeds a predetermined of the bimetallic element 2| decreases below a predetermined value, the two arms of the bimetallic element move toward each other sum- 'ciently to cause the movable contact 2| to be snapped out of engagement with contact 22 and into engagement with contact 22. The setting of the thermostatic device H is arranged to be manually adjusted by manually adjusting the position of the stop 20 in any suitabale manner.

In addition to the ambient temperature in which the bimetallic element 2| is located, it is also subject to a local heating source comprising a heating element II which is arranged to be connected across the secondary terminals II and II of the transformer II when the contacts 2| and 22 of the switch 2| are in engagement.

With the arrangement heretofore described, it will be evident to those skilled in the art that under normal operating conditions the bimetallic element 25 operates to elect periodic engagement and disengagement of the contacts 2| and 22 in accordance with the ambient temperature to maintain a predetermined thermal condition of the blanket II.

In order to protect the blanket ll against abnormally high temperatures in case it becomes folded or rolled while in use, I provide in accordance with my invention a thermally controlled device 3|, which is so constructed and connected as to distinguish betweenan abnormally high blanket temperature due merely to an increase in the supply circuit voltage and a similar abnormally high blanket temperature produced by an abnormal operating condition of the blanket while in use. This control device II depends for its operation upon the increase in the resistance of the heating circuit of the blanket resulting from abnormally high temperatures of the blanket produced by other abnormal operating conditions than abnormally high voltage conditions of the supply circuit.

In the preferred embodiment of the thermally controlled device 2| shown in the drawing. I accomplish the desired results by providing two identical bimetallic strips I2 and 2| having two of their corresponding expansion metals adjacent to each other and with one end of each strip fastened to a stationary block 34 while the free ends of these strips are hinged to a bar ll carrying a contact 30. With such a construction, it will be apparent that changes in ambient temperature do not change the position of the bar ll and the contact 30 since the forces exerted by the bimetallic strips 22 and 33 due to changes in ambient temperature are equal and opposite and, therefore, neutralize each other. Heating elements 3! and 2! are respectively provided for the bimetallic elements 32 and 23. The heating element 31 comprises a relatively large number of turns of fine resistance wire connected between the secondary terminal ll of the transformer l1 and the stationary contact 22 of the thermally operated switch 2. so that the heating element 21 is connected across the secondary terminals II and it of the transformer II when the contacts 2| and 22 of the switch 2| are in engagement. The heating element 32 comprises a relatively small number of turns of larger resistance wire connected in series between the contacts ll of the plug l5 and the contact 22 of the thermally operated switch 2. so that the heating element 3! is connected in series with the heating element i2 of the electric blanket I I. Preferably, the heating element II is noninductively woundsoastodecreasetheimpedanceofthe heating element to a minimum.

Theheatingelementllisdesigned sothatits temperature coeilicient of resistivity is substantially equal to the temperature coeillcient of resistivity of the series circuit containing the heating element ll of the thermally operated control device SI and the heating element 12 of the blanket ll under normal operating conditions of the blanket. Also, the current-resistance drops across the two heating elements I! and ll are made substantially the same under normal operating conditions of the blanket ll so that both of these heating elements produce substantially the same amount of heat. Therefore, changes in the voltage of the supply circuit ll produce no movement of the bar ll and the contact 2.. When, however, an abnormally high blanket temperature occurs due to the blanket being folded or rolled while in use or some other abnormal operating condition occurs which does not produce a similar effect on the current through the heating element 81, the increase in resistance of the heating element 12 of the blanket ll due to the abnormally high blanket temperature effects a reduction in the current through the heating element It without a similar reduction in current being produced through the heating element 21. Consequently. the balance of the opposing forces exerted by the bimetallic strips 82 and II is upset and the bar II is moved so as to cause the movable contact II to engage the stationary contact II. The contact 30 is connected to the contact 2! of the switch 2| through a current limiting resistor ll, and the contact 2! is connected to the intermediate taps II and 42, respectively, on the heating elements 21 and II. Consequently, when the contacts 88 and I. are in engagement and the thermally operated switch 2| is in its open position in which the heating element l2 of the electric blanket is disconnected from the supply circuit ll, heating circuits are completed for portions of the heating elements II and 31 across the secondary terminals II and W of the transformer ll so that the heating element ll produces sumcient heat to maintain the switch 2| in its open position and the heating element 31 produces sufllcient heat to maintain the control device II in its operated position under normal operating voltage conditions of the supply circuit ll. Therefore. the normal heating circuit for the heating element l2 through the contacts 2| and 22 of the switch 2| is permanently opened so as to reduce the current supplied to the heating element l2 to zero and thereby cause the temperature of the blanket to be reduced.

Normal operating conditions of the temperature control system may be restored after the cause of the abnormal blanket temperature has been removed by merely opening the switch II in the primary circuit of the transformer l1 and maintaining it open for a sufficient length of time to allow the thermally operated devices 22 and II and the blanket l2 tobe restored totheir normal operating conditions.

While I have, in accordance with the patent statutes, shown and described my invention as applied to a particular system and as embodying various devices diagrammatically indicated, changes and modifications will be obvious to those skilled in the art, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

oi current, a thermal responsive device having one position in which it connects said source to said heating circuit and another position in which said connection is interrupted, a heating element for said thermal responsive device connected to said source, a second thermal-responsive device having two similar opposed thermostatic elements, a heating element for one of said thermostatic elements connected across said source in parallel with said heating circuit, a heating element for the other of said thermostatic elements connected in series with said heating circuit, and means controlled by second thermal responsive device for increasing for a given voltage of said source the heating efiect produced. by the heating element oi said first mentioned thermal responsive device and the first mentioned heating element of said second thermal responsive device.

2. In combination, an electric heating device having a heating circuit the impedance of which varies with the temperature of the device, a source of current, a thermal responsive device having one position in which it connects said source to said heating circuit and another position in which said connection is interrupted, a heating element for said thermal responsive device connected to said source, a second thermal responsive device having two similar oppo ed thermostatic elements, a heating element for one oi said thermostatic elements connected across said source in parallel with said heating circuit, a heating element i'orthe other of said thermostatic elements connected in series with said heating circuit, and means controlled by second thermal responsive device for changing the connections of the heating element of said first men tioned thermal responsive device to increase the heating efiect produced thereby under normal operating voltage conditions of said source whereby said first mentioned thermal responsive de-' vice remains permanently in said position in which said connection to said heating circuit is interrupted.

3. In combination, an electric heating device having a heating circuit the impedance 01' which varies with the temperature of the device, a source 01' current, a thermal responsive device having one position in which it connects said source to said heating circuit and another position in which said connection is interrupted, a heating element for said thermal responsive device connected to said source, a second thermal responsive device having two similar opposed thermostatic elements. a heating element for one of said thermostatic elements connected across said source in parallel with said heating circuit, a heating element for the other of said thermostatic elements connected in series with said heating circuit, and means jointly controlled by both of said thermal responsive devices only upon the occurrence of an abnormally high temperature of said heating device while said first mentioned thermal responsive device is in said position in which said connection to said heating circuit is interrupted for increasing for a given voltage of said source the heating efi'ect produced by the heating element of said first mentioned thermal responsive device so that said first mentioned thermal responsive device remains permanently in said position in which said connection to said heating circuit is interrupted.

4. In combination, an electric heating device having a heating circuit the impedance of which varies with the temperature of the device, a source 01 current, a thermal responsive device having one position in which it connects said source to said heating circuit and another position in which said connection isinterrupted, a heating element for said thermal responsive device connected to said source, a second thermal responsive device having two similar opposed thermostatic elements, a heating element for one of said thermostatic elements connected across said source in parallel with said heating circuit, a heating element for the other of said thermostatic elements connected in series with said heating circuit, and means jointly controlled by both of, said thermal responsive devices only upon the occurrence of an abnormally high temperature of said heating device while said first mentioned thermal responsive device is in said position in which said connection to said heating circuit is interrupted for increasing for a given voltage oi said source the heating efiect produced by the heating element of said first mentioned thermal responsive device and the heating eiIect produced by the first mentioned heating element of said second thermal responsive device.

5. In combination, an electric heating device having a heating circuit the impedance of which varies with the temperature of the device, a source 01' current, a thermal responsive device having one position in which it connects said source to said heating circuit an another position in which said connection is interrupted. a heating element for said thermal responsive device connected to said source, a second thermal responsive device having two similar opposed thermostatic elements, a heating element for one of said thermostatic elements connected across said source in parallel with said heating circuit, a heating element for the other 01' said thermostatic elments connected in series with said heating circuit, andmeans iointly controlled by both oi said thermal responsive devices only upon the occurrence of an abnormally high temperature of said heating device while said first mentioned thermal device is in said position in which said connection to said heating circuit is interrupted for changing the connections of the heating element of said first mentioned thermal responsive device and of the first mentioned heating element 01' said second mentioned thermal responsive device to increase the heating efi'ects produced thereby for a given voltage of said source so that said first mentioned thermal responsive device remains permanently in said position in which said connection to said heating circuit is interrupted.

6. An arrangement for protecting an electrically operated device against abnormally high temperatures comprising two differentially arranged devices, means for causing one of said differentially arranged devices to exert a torque proportional to the current flowing through said electrically operated device, and means for causing the other of said differentially arranged devices to exert a torque proportional to the voltage of the electric source supplying current to said electrically operated device and independently of changes in the impedance of said electrically operated device.

7. An arrangement for protecting an electric heating device against abnormally high temperatures comprising two opposed thermostatic clemeats, means for heating one 0! said elements 8. An arrangement for protecting an electric m bianket against abnormally high temperatures due to an abnormal use of the blanket comprising two opposed thermostatic elements, a heating element for one of said thermostatic elements connected in series relation withethe heating element oi said blanket and s heating element for the other thermostatic element connected in paraliei with said first mentioned heating element and the heating element of said blanket.

WILLIAM K. KEARBLIY. 

